RFFC2071A Datasheet, PDF (7/22 Pages) RF Micro Devices – 2.7GHz RF SYNTHESIZER/VCO WITH INTEGRATED RF MIXER

RFFC2071A/2A
External Reference
The RFFC2071A and RFFC2072A have been designed to use an external reference such as a TCXO. The typical input will be a
0.8Vp-p clipped sine wave, which should be AC-coupled into the reference input. When the PLL is not in use, it may be desir-
able to turn off the internal reference circuits, by setting the REFSTBY bit low, to minimize current draw while in standby mode.
On cold start, or if REFSTBY is programmed low, the reference circuits will need a warm-up period. This is set by the SU_WAIT
bits. This will allow the clock to be stable and immediately available when the ENBL bit is asserted high, allowing the PLL to
assume normal operation.
If the current consumption of the reference circuits in standby mode, typically 2mA, is not critical, then the REFSTBY bit can be
set high. This allows the fastest startup and lock time after ENBL is taken high.
Wideband Mixer
The mixers are wideband, double-balanced Gilbert cells. They support RF/IF frequencies from 30MHz up to 2700MHz. Each
mixer has an input port and an output port that can be used for either IF or RF (in other words, for up- or down-conversion). The
mixer current can be programmed to between about 15mA and 45mA depending on linearity requirements. The majority of the
mixer current is sourced through the output pins via either a center-tapped balun or an RF choke in the external matching cir-
cuitry to the supply.
The RF mixer input and output ports are differential and require baluns and simple matching circuits optimized to the specific
application frequencies. A conversion gain of approximately -2dB (not including balun losses) is achieved with 100 differen-
tial input impedance, and the outputs driving 200 differential load impedance. Increasing the mixer output load increases
the conversion gain.
The mixer has a broadband common gate input. The input impedance is dominated by the resistance set by the mixer 1/gm
term, which is inversely proportional to the mixer current setting. The resistance will be approximately 85 at the default mixer
current setting (100). There is also some shunt capacitance at the mixer input, and the inductance of the bond wires (about
0.5nH on each pin) to consider at higher frequencies. The following diagram is a simple model of the mixer input impedance:
0.5nH
RFFC207x
Mixer Input
0.5nH
0.5pF
Rin
Typ 85
The mixer output is high impedance, consisting of approximately 2k resistance in parallel with some capacitance, approxi-
mately 1pF. The mixer output does not require a conjugate matching network. It is a constant current output which will drive a
real differential load of between 50Ω and 500Ω, typically 200Ω. Since the mixer output is a constant current source, a higher
resistance load will give higher output voltage and gain. A shunt inductor can be used to resonate with the mixer output capac-
itance at the frequency of interest. This inductor may not be required at lower frequencies where the impedance of the output
capacitance is less significant. At higher output frequencies the inductance of the bond wires (about 0.5nH on each pin)
becomes more significant. The following diagram is a simple model of the mixer output:
0.5nH
1K
1K
1pF
0.5nH
RFFC207x
Mixer Output
DS140110
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